1. Field of the Invention
The present invention relates to a group 13 nitride crystal, and a group 13 nitride crystal substrate.
2. Description of the Related Art
Semiconductor materials based on gallium nitride (GaN) are used for blue color LED (light-emitting diode) or white color LED, and a semiconductor device such as semiconductor laser (also called “LD: Laser Diode”). The white color LED is used for illumination purpose or back lighting of cell phones, LC (Liquid Crystal) display or the like. The blue color LED is used for traffic lights or other illumination purpose and so on. On the other hand, blue-violet semiconductor laser is used as light sources of Blu-ray discs. Presently, most of semiconductor devices based on GaN used as light sources of UV or violet-blue-green color are fabricated by using MO-CVD (Metalorganic Chemical Vapor Deposition) method or MBE (Molecular Beam Epitaxy) method to grow crystal on sapphire or SiC substrate.
There are problems in a case that sapphire or SiC is used as substrate. Crystal defects increases because of the significant difference of thermal expansion coefficient or lattice constant between the substrate and the group 13 nitride. Such a defect affects device properties. For example, it becomes harder to elongate the lifetime of emitting device. And, the operating power may increase. In order to address these problems, it is the most preferable to use a gallium nitride substrate which is made of the same material as crystal to be grown on the substrate.
Presently, free-standing GaN substrates are manufactured in such a manner that a thick gallium nitride crystal is grown on a hetero-substrate such as sapphire substrate or GaAs substrate by HVPE (Hydride Vapor Phase Epitaxy) with employing ELO (Epitaxial Lateral Overgrowth) which is a method to reduce the dislocation density, and then the thick film of gallium nitride is separated from the hetero-substrate. The gallium nitride substrate manufactured as such has a dislocation density reduced to the order of 106 cm−2, and allows a size up to 2 inches in practical use mainly for laser device purpose. Recently, there is a further need for much larger diameter of substrate up to 4 inches or 6 inches for electronic devices, or cost saving of white color LEDs.
Warpage or cracks which may be induced by the difference of the thermal expansion coefficient or the lattice constant between the hetero-substrate and the gallium nitride hinders to enlarge the diameter of substrate. The aforementioned dislocation density still remains. There is also a problem of high manufacturing cost in processes of separating one thick film of gallium nitride from one hetero-substrate, and polishing it to form the gallium nitride substrate.
On the other hand, as one of liquid phase methods to realize the gallium nitride substrate, many efforts have been made for developing the flux method in which the gallium nitride crystal is formed by dissolving the nitrogen from a gaseous phase into a molten mixture of group 13 metal and alkali metal.
In the flux method, a molten mixture containing the alkali metal such as sodium (Na) and potassium (K) and the group 13 metal such as gallium (Ga) is heated to about 600 to 900 degrees Celsius under an atmosphere where the nitrogen pressure is 10 MPa or less. Thus, the nitrogen is dissolved from the gaseous phase and reacts with the group 13 metal in the molten mixture to form the group 13 nitride crystal. The flux method allows a crystal growth with a lower temperature and lower pressure in comparison with ether liquid phase methods. The crystal formed by the flux method has a low dislocation density advantageously lower than 106 cm−2.
There is a report that gallium nitride crystal is formed under conditions that sodium azide (NaN3) and metal Ga which are used as source materials are put and sealed in a reactor vessel made of stainless steel (as for sizes inside of the vessel, inner diameter is 7.5 mm, length is 100 mm) under a nitrogen atmosphere, and the reactor vessel is retained at 600 to 800 degrees Celsius for 24 to 100 hours (Chemistry of Materials Vol. 9 (1997) 413-416).
Japanese Patent Application Laid-open No. 2008-94704 discloses a method of manufacturing a column-like crystal of gallium nitride by using a needle-like crystal of aluminum nitride (AlN) as seed crystal in order to provide a large crystal of gallium nitride. Japanese Patent Application Laid-open No. 2006-045047 discloses a method of manufacturing a needle-like crystal of aluminum nitride which becomes a seed crystal. Japanese Patent Application Laid-open No. 2009-126771 discloses a seed crystal of which a yellow emission effect is observed, and a gallium nitride crystal which is formed on the seed crystal and has a crystal layer of which a yellow emission effect is not observed.
In a case that gallium nitride crystal is formed from aluminum nitride as seed crystal, however, the difference of lattice constant between aluminum nitride and gallium nitride may arise dislocations due to the lattice mismatch. Since thermal expansion coefficient is also different between aluminum nitride and gallium nitride, thermal stress may arise new dislocations or even cracks, in the course of cooling from a crystal growth temperature to a room temperature.
Therefore, it is preferable to use gallium nitride crystal as seed crystal which has the same lattice constant or the same thermal expansion coefficient with the target crystal, in order to grow a high quality gallium nitride crystal with low dislocation density. However, it is difficult to grow a needle-like crystal of gallium nitride by the method disclosed in Japanese Patent Application Laid-open No. 2006-045047. Therefore, it is difficult to obtain a high quality bulk crystal by any conventional method.